The present invention relates to a metallic surface treating agent for metallic substrates, particularly precoated (hereinafter referred to briefly as PCM) steel panels for use as metal-coated steel plates such as the zinc-coated steel panel, aluminum-coated steel panel, tin-coated steel panel, etc., a method for surface treatment of a PCM steel panel, a method of producing a PCM steel panel, and a PCM steel panel as produced by said method.
As the metallic surface treating agent, a chromium-containing surface treating agent such as a chromate system or a phosphate-chromate system has heretofore been used broadly and still in use today. However, in view of the recent trend toward more stringent regulatory control for environmental protection, it is likely that the use of such coating systems will be restricted for fear of the toxicity, particularly carcinogenicity, of chromium. Therefore, development of a rust-preventing agent not containing chromium and yet as effective as the chromating agent in imparting corrosion resistance has been awaited. As disclosed in Japanese Kokai Publication Hei-11-29724, the inventors of the present invention previously developed a nonchromate rust-preventing agent comprising a water-base resin and, as incorporated therein, a thiocarbonyl group-containing compound, a phosphate ion, and water-dispersible silica. Regrettably, however, this system was found to be deficient in storage stability and somewhat poor in corrosion resistance at thin coating thickness. Meanwhile, with regard to silane coupling agents, an acidic surface treating agent containing two dissimilar silane coupling agents is disclosed in Japanese Kokai Publication Hei-8-73775. However, this system is intended to improve finger-print resistance and overcoat adhesion and is quite deficient in corrosion resistance for use in applications where high corrosion resistance and good processability are required after such metallic surface treatment as in the present invention. Moreover, Japanese Kokai Hei-10-60315 discloses a steel structure surface treating agent, which contains a silane coupling agent having a certain functional group reactive with an aqueous emulsion, but the corrosion resistance required here is only that of a degree satisfying comparatively mild test requirements such as those of wet tests and as far as corrosion resistance is concerned, the system is a far cry from a rust-preventing agent system as provided by the present invention. With the above state of the art byway of background, there has been a standing demand for development of a rust-preventing agent expressing sufficient corrosion resistance and overcoat adhesion at thin coating thickness.
Meanwhile, in the field of PCM steel panels, too, the advent has been awaited of a PCM steel panel produced by using a nonchromate rust-preventing agent and yet expressing overcoat adhesion and corrosion resistance comparable or superior to those of the conventional chromated PCM steel panel.
The present invention has for its object to provide a nonchromate metallic surface treating agent for PCM use which is suited for metal-coated steel panels, particularly galvanized steel panels and is capable of imparting high processability and corrosion resistance to PCM steel panels, with the additional advantage of improved storage stability.
The nonchromate metallic surface treating agent for PCM use according to the present invention comprises, in each liter thereof,
(a) a silane coupling agent and/or a hydrolytic condensation product thereof in an amount of 0.01 to 100 g/l,
(b) water-dispersible silica in an amount of 0.05 to 100 g/l (solids), and
(c) a zirconium compound in an amount of 0.01 to 50 g/l in terms of zirconium ion and/or a titanium compound in an amount of 0.01 to 50 g/l in terms of titanium ion.
The nonchromate metallic surface treating agent for PCM use according to the present invention may contain one or more members selected from among sulfide ion, thiosulfate ion, persulfate ion and a triazinethiol compound in an amount of 0.01 to 100 g/l.
The method of treating a metallic surface according to the present invention comprises treating the surface of a metal-coated steel panel with any of the above metallic surface treating agents, and is most suited for the surface treatment of zinc-coated steel panels.
The method of producing a PCM steel panel according to the present invention comprises treating the surface of a metal-coated steel panel with any of said metallic surface treating agents and applying a chromium-free primer and a top coating serially thereon.
The PCM steel panel of the present invention is obtainable by the above production method.